High-strength spring steel having excellent wire-rod rolling properties

ABSTRACT

The present invention relates to a high-strength spring steel having excellent wire-rod rolling properties, consisting essentially of, in terms of mass %: C: 0.40% to 0.65%; Si: 1.20% to 2.80%; Mn: 0.30% to 1.20%; P: 0.020% or less; S: 0.020% or less; Cu: 0.40% or less; Ni: 0.80% or less; Cr: 0.70% or less; Ti: 0.060% to 0.140%; Al: 0.10% or less; N: 0.010% or less; and O: 0.0015% or less, and optionally: B: 0.0005% to 0.0050%, with the remainder being Fe and inevitable impurities, in which the contents in terms of mass % of the specified chemical components satisfy the following Expressions (1) to (3): 
         X 1=0.14×[Si]−0.11×[Mn]−0.05×[Cu]−0.11×[Ni]−0.03×[Cr]+0.02≦0.2   Expression (1)
 
         X 2=(α−500)/β≧3.0   Expression (2)
 
       α=912−231×[C]+32×[Si]−20×[Mn]−40×[Cu]−18×[Ni]−15×[Cr]
 
       β=10̂(0.322−0.538×[C]+0.018×[Si]+1.294×[Mn]+0.693×[Cu]+0.609×[Ni]+0.847×[Cr])
 
         X 3=31×[C]+2.3×[Si]+2.3×[Mn]+1.25×[Cu]+2.68×[Ni]+3.57×[Cr]−6×[Ti]≧24.0   Expression (3).

FIELD OF THE INVENTION

The present invention relates to a high-strength spring steel havingexcellent wire-rod rolling properties.

BACKGROUND OF THE INVENTION

On the occasion of seeking weight reduction of suspension springs inanswer to demands for weight reduction of vehicles (automobiles),development of springs allowing high design stress has been pursued. Forthe purpose of heightening the design stress of springs, it is requiredto engineer improvements in various characteristics of the springs, andmore specifically, it is essential to add alloy elements. For example,it may be thought to add Si when improvement in settling property isintended, while it may be thought to add such an element as Cu, Ni or Crwhen improvement in corrosion resistance is intended.

By the way, increases in amounts of alloy elements added with the aim ofimproving spring characteristics tend to yield detriments such asoccurrence of ferrite decarburization and formation of bainite duringthe cooling after wire-rod rolling. The former detriment is fatal forthe springs to which shot peening is to be given, while the latterdetriment may become a harmful factor at the time of secondary working,and hence it becomes important to avoid both of these detriments. As thetechniques to avoid both the detriments, there have been knowntechniques described e.g. in the following Patent Documents 1 and 2.

The following Patent document 1 has disclosed the technique of heating asteel material at a temperature of 1,170° C. or more for at least 2minutes under hot rolling, cooling the material at an average coolingrate of 5 to 300° C./min in a temperature range from 750° C. to 600° C.after the rolling, and further adopting descaling process. The followingPatent Document 2 has disclosed the technique of subjecting a steelmaterial to hot rolling in a condition that, after heating furnaceextraction, the temperature before finishing is set to less than 1,000°C., keeping the steel material in a temperature range of 1,000° C. to1,150° C. for 5 seconds or less after finish rolling and then winding itup, thereafter cooling the wound steel material to a temperature of 750°C. or less at a cooling rate of 2 to 8° C./sec, and further graduallycooling down to 600° C. by spending 150 seconds or more after thewinding-up.

Patent Document 1: Japanese Patent No. 4031267

Patent Document 2: Japanese Patent No. 5330181

SUMMARY OF THE INVENTION

However, each of the techniques disclosed in Patent Documents 1 and 2requires execution of individually specified rolling process.Accordingly, it has been desired to avoid occurrence of ferritedecarburization and formation of bainite by adjusting chemicalcomponents of a steel material instead of adopting a technique ofproviding a specific rolling process, thereby developing a high-strengthspring steel having excellent wire-rod rolling properties.

The present invention has been made against a background of theforegoing circumstances, and an object of the present invention is toprovide a high-strength spring steel having excellent wire-rod rollingproperties by adjusting chemical components of a steel material to avoidoccurrence of ferrite decarburization and formation of bainite.

Namely, the present invention relates to the following items 1 to 3.

1. A high-strength spring steel having excellent wire-rod rollingproperties, consisting essentially of, in terms of mass %:

C: 0.40% to 0.65%;

Si: 1.20% to 2.80%;

Mn: 0.30% to 1.20%;

P: 0.020% or less;

S: 0.020% or less;

Cu: 0.40% or less;

Ni: 0.80% or less;

Cr: 0.70% or less;

Ti: 0.060% to 0.140%;

Al: 0.10% or less;

N: 0.010% or less; and

O: 0.0015% or less,

and optionally:

B: 0.0005% to 0.0050%,

with the remainder being Fe and inevitable impurities,

in which the contents in terms of mass % of the specified chemicalcomponents satisfy the following Expressions (1) to (3):

X1=0.14×[Si]−0.11×[Mn]−0.05×[Cu]−0.11×[Ni]−0.03×[Cr]+0.02≦0.2  Expression (1)

X2=(α−500)/β≧3.0   Expression (2)

α=912−231×[C]+32×[Si]−20×[Mn]−40×[Cu]−18×[Ni]−15×[Cr]

β=10̂(0.322−0.538×[C]+0.018×[Si]+1.294×[Mn]+0.693×[Cu]+0.609×[Ni]+0.847×[Cr])

X3=31×[C]+2.3×[Si]+2.3×[Mn]+1.25×[Cu]+2.68×[Ni]+3.57×[Cr]−6×[Ti]≧24.0  Expression (3).

2. The high-strength spring steel having excellent wire-rod rollingproperties according to item 1, having a 400° C.-temper hardness of 53.0HRC or more.

3. The high-strength spring steel having excellent wire-rod rollingproperties according to items 1 or 2, having a crystal grain size numberof 9 or more.

The present inventors have found that it is possible to formulate (asExpression (1)) the relation between a ferrite decarburization depth anda parameter (X1) determined by converting the degrees of contributionsto the depth from respective chemical components of a steel materialinto numerical values, formulate (as Expression (2)) the relationbetween bainite formation in the case of cooling at a normal coolingrate after wire-rod rolling and a parameter (X2) determined byconverting the degrees of contributions to the bainite formation fromrespective chemical components of a steel material into numerical valuesand formulate (as Expression (3)) the relation between hardness in thecase of subjecting tempering treatment at 400° C. and a parameter (X3)determined by converting the degrees of contributions to the hardnessfrom respective chemical components of a steel material into numericalvalues. Namely, it is possible to obtain a high-strength spring steelhaving excellent wire-rod rolling properties by adjusting contents ofchemical components in a steel material so as to satisfy the foregoingExpressions (1) to (3).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph for illustrating conditions of Expression (1).

FIG. 2 is a graph for illustrating conditions of Expression (2).

FIG. 3 is a graph for illustrating conditions of Expression (3).

FIG. 4 is a graph for showing the reason for setting the lower limit ofTi content to 0.060 mass %.

DETAILED DESCRIPTION OF THE INVENTION

The following are descriptions of reasons and conditions for limitingindividual chemical components (elements) in the composition of thepresent high-strength spring steel. Incidentally, the content of eachcomponent is shown in terms of mass %, and “mass %” is the same as “wt%”.

-   (1) C: 0.40% to 0.65%

C is an element essential for spring steel to secure strength. When theC content is lower than 0.40%, it is impossible to achieve the intendedspring strength. On the other hand, when C is added in an amountexceeding 0.65%, degradation of tenacity and fatigue characteristics iscaused, and hence the upper limit of C content is set to 0.65%. The Ccontent is preferably from 0.45% to 0.60%.

-   (2) Si: 1.20% to 2.80%

Si is an element effective in enhancing settling resistance of springsteel. Si is therefore added in an amount of 1.20% or more. However,addition of Si in excess of 2.80% tends to cause not only degradation ofsettling properties but also occurrence of ferrite decarburization, andhence the upper limit of Si content is set to 2.80%. The Si content ispreferably more than 1.50% and 2.50% or less, more preferably more than2.00% and 2.50% or less.

-   (3) Mn: 0.30% to 1.20%

Mn functions as an ingredient for fixing S, which is a tenacitydegrading element, in the form of MnS. Mn functions also as a quenchingproperty improver. In order to make good use of these functions, Mn isadded in an amount of 0.30% or more. However, addition of Mn in anamount exceeding 1.20% results in degradation of tenacity, and hence theupper limit of Mn content is set to 1.20%. The Mn content is preferablymore than 0.50% and 1.10% or less, more preferably less than 1.00%.

-   (4) P: 0.020% or less

Since P makes crystal grain boundaries brittle, the content thereof isrequired to be minimized. So long as the P content is 0.020% or less,impact of reduction in strength of the grain boundaries is slight, whileextreme reduction in P content is undesirable from the industrialviewpoint because it brings about elongation of smelting process whichresults in an increased cost.

-   (5) S: 0.020% or less

S is inevitably present in steel and, as mentioned above, combines withMn to form MnS inclusions which become starting points of stressconcentration. Unduly high S content not only increases the amount ofMnS inclusions but also causes reduction in fatigue strength. However,so long as the S content is 0.020% or less, reduction in fatiguestrength is exceedingly slight.

-   (6) Cu: 0.40% or less

Cu is an element effective in improving corrosion resistance. Inaddition, it is also effective in preventing ferrite decarburization.The Cu content is preferably from 0.20% to 0.37%.

-   (7) Ni: 0.80% or less

Ni is an element effective in improving corrosion resistance. Inaddition, it is also effective in preventing ferrite decarburization.Incorporation of Ni, however, brings about an increase in cost, andhence the upper limit of Ni content is set to 0.80%. The Ni content ispreferably from 0.50% to 0.75%.

-   (8) Cr: 0.70% or less

Cr is an element effective in improving corrosion resistance. Inaddition, it is also effective for adjustment of quenching properties.Excessive addition of Cr causes formation of sharp corrosion pits, andhence the upper limit of Cr content is set to 0.70%. The Cr content ispreferably from 0.20% to 0.50%.

-   (9) Ti: 0.060% to 0.140%

Ti is an element that is apt to form carbide. Ti-based carbidescontribute to fining of crystal grains and enhance a fatiguecharacteristic, a delayed fracture characteristic and settlingresistance. For these reasons, Ti is added in an amount of 0.060% ormore. When the Ti content exceeds 0.140%, however, the effects of Tiaddition become saturated; on the contrary, deterioration in rollingproperties is brought about. The upper limit of Ti content is thereforeset to 0.140%. The Ti content is preferably from 0.080% to 0.120%.Reasons why the lower limit of Ti content is set to 0.060% will bedescribed later.

-   (10) Al: 0.10% or less

Al is an element that acts as a deoxidizer during liquid steeltreatment. However, when Al is added in an amount exceeding 0.10%,inclusions are increased, whereby lowering of fatigue strength is rathercaused. The upper limit of Al content is therefore set to 0.10%.

-   (11) N: 0.010% or less

N combines with Ti to form nitride, resulting in lowering of fatiguestrength. The upper limit of N content is therefore set to 0.010%.

-   (12) O: 0.0015% or less

Since O forms oxide-based inclusions, the content thereof is set to0.0015% or less.

-   (13) Remainder: Fe and inevitable impurities

Incidentally, descriptions of Fe and inevitable impurities are omittedin Table 1.

-   (14) Satisfying the following Expression (1)

X1=0.14×[Si]−0.11×[Mn]−0.05×[Cu]−0.11×[Ni]−0.03×[Cr]+0.02≦0.2  Expression (1)

In order to examine the adequacy of Expression (1), simulations offerrite decarburization were conducted. In the simulations, steelsamples having chemical compositions shown in Table 1, respectively,were each independently melt-formed and hot-rolled into bars having 22mmφ. Thereafter, these samples were machined into bars having dimensionsof 14 mmφ×20 mm, subjected to heat treatment with the condition thatthey were kept at 900° C. for 100 minutes, and then oil-cooled.Subsequently thereto, ferrite decarburization depth measurements weremade on the samples after the heat treatment. Measurement resultsobtained are shown in Table 1 and FIG. 1.

TABLE 1 Ferrite Presence or Absence Presence or Absence Decarbu- ofFerrite of Bainite 400° C. Chemical Components (mass %) rizationDecarburization during Formation during Temper C Si Mn Ti B Expressions(1) to (3) Depth Rod-wire Rolling Rod-wire Rolling Hardness CrystalGrain Steel 0.40- 1.20- 0.30- Cu Ni Cr 0.060- 0.0005- X1 X2 α β X3 (mm)0.5° C./s 0.5° C./s 1.5° C./s (HRC) Size Number Species 0.65 2.80 1.20≦0.40 ≦0.80 ≦0.70 0.140 0.0050 ≦0.2 ≧3.0 — — ≧24.0 — — — — ≧53.0 ≧9.0Ex. 1 0.53 2.10 0.72 0.27 0.58 0.33 0.092 0.0011 0.148 4.72 816 67.025.4 0.062 absent absent absent 53.9 9.6 Ex. 2 0.52 2.02 0.65 0.32 0.710.33 0.090 0.0015 0.127 4.39 813 71.3 25.2 0.061 absent absent absent53.4 9.5 Ex. 3 0.50 2.15 0.78 0.27 0.57 0.33 0.093 0.0017 0.149 3.94 82482.2 24.7 0.063 absent absent absent 53.5 9.7 Ex. 4 0.50 2.09 0.70 0.320.72 0.33 0.091 0.0010 0.131 3.69 819 86.3 24.9 0.056 absent absentabsent 53.1 9.6 Ex. 5 0.49 2.19 0.83 0.27 0.58 0.33 0.094 0.0018 0.1483.32 826 98.1 24.6 0.064 absent absent absent 53.0 10.1 Ex. 6 0.48 2.140.75 0.32 0.72 0.33 0.090 0.0017 0.132 3.15 824 102.9 24.5 0.060 absentabsent absent 53.0 9.5 Ex. 7 0.52 2.09 0.76 0.30 0.49 0.25 0.092 0.00160.153 5.28 819 60.4 24.7 0.052 absent absent absent 53.6 9.5 Ex. 8 0.522.10 0.76 0.30 0.50 0.25 0.124 0.0017 0.153 5.20 819 61.3 24.6 0.049absent absent absent 53.4 11.2 Ex. 9 0.56 2.11 0.76 0.30 0.50 0.25 0.1220.0014 0.154 5.31 810 58.4 25.8 0.055 absent absent absent 53.5 10.8 Ex.10 0.56 1.79 0.39 0.20 0.40 0.25 0.089 0.0016 0.166 22.09 813 14.2 24.10.056 absent absent absent 53.4 9.7 Ex. 11 0.52 2.09 0.76 0.30 0.50 0.250.093 0 0.152 5.20 819 61.3 24.7 0.051 absent absent absent 53.5 9.4 Ex.12 0.48 2.11 0.75 0.32 0.72 0.33 0.091 0 0.128 3.14 823 102.8 24.4 0.048absent absent absent 53.3 9.5 Comp. 0.48 2.01 0.60 0.37 0.65 0.15 0.0910 0.141 7.17 825 45.3 23.1 0.057 absent absent absent 52.1 9.7 Ex. 1Comp. 0.52 2.13 0.76 0.30 0.50 0.25 0.004 0 0.157 5.21 820 61.4 25.40.066 absent absent absent 53.5 7.9 Ex. 2 Comp. 0.52 2.09 0.76 0.30 0.500.25 0.032 0.0013 0.152 5.20 819 61.3 25.1 0.063 absent absent absent53.8 8.2 Ex. 3 Comp. 0.52 2.37 0.35 0.29 0.37 0.23 0.081 0.0020 0.25123.51 839 14.4 24.1 0.163 present absent absent 53.4 9.3 Ex. 4 Comp.0.56 2.39 0.39 0.20 0.43 0.25 0.090 0.0020 0.247 21.89 832 15.2 25.50.116 present absent absent 54.1 9.4 Ex. 5 Comp. 0.48 2.39 0.40 0.200.40 0.25 0.089 0.0019 0.249 21.21 851 16.5 23.0 0.158 present absentabsent 52.0 9.5 Ex. 6 Comp. 0.61 2.39 0.40 0.20 0.40 0.25 0.092 0.00170.249 22.78 821 14.1 27.0 0.126 present absent absent 55.6 9.7 Ex. 7Comp. 0.57 2.20 0.40 0.20 0.40 0.25 0.091 0.0015 0.223 22.07 824 14.725.3 0.109 present absent absent 54.2 9.8 Ex. 8 Comp. 0.56 2.78 0.400.20 0.40 0.25 0.090 0.0017 0.304 22.65 845 15.2 26.3 0.156 presentabsent absent 55.2 9.6 Ex. 9 Comp. 0.42 2.64 1.11 0.31 0.21 0.25 0.0970.0020 0.221 2.62 857 136.3 22.9 0.088 present absent present 52.2 10.6Ex. 10 Comp. 0.41 2.56 0.96 0.29 0.21 0.14 0.091 0.0020 0.231 5.27 86368.7 21.7 0.114 present absent absent 51.5 9.6 Ex. 11 Comp. 0.44 2.500.98 0.28 0.78 0.58 0.087 0.0016 0.145 0.93 837 362.0 25.6 0.048 absentpresent present 54.4 9.4 Ex. 12 Comp. 0.44 2.52 0.73 0.30 0.80 0.590.079 0.0015 0.172 1.83 841 186.2 25.3 0.063 absent present present 53.29.3 Ex. 13 Comp. 0.41 2.10 1.01 0.24 0.24 0.36 0.088 0 0.154 2.98 845115.8 21.6 0.087 absent absent present 51.5 9.5 Ex. 14 Comp. 0.40 2.180.84 0.26 0.17 0.12 0.001 0 0.198 8.60 857 41.5 20.6 0.064 absent absentabsent 50.1 7.6 Ex. 15 Comp. 0.60 1.97 0.83 0.10 0.11 0.12 0.003 0 0.18414.05 812 22.2 25.9 0.072 absent absent absent 53.2 7.4 Ex. 16 Comp.0.54 1.29 0.74 0.09 0.06 0.63 0.002 0 0.089 6.79 800 44.1 23.9 0.031absent absent absent 52.4 7.9 Ex. 17

FIG. 1 is a graph made by plotting the coordinate data from every steelspecies with ferrite decarburization depth as vertical axis and X1 inExpression (1) as horizontal axis. X1 includes a polynomial formed byperforming addition or subtraction of component terms each of which isobtained by multiplying each of the contents of the specified chemicalcomponents (Si, Mn, Cu, Ni and Cr) by the individually specifiedcoefficient and, as clearly seen from FIG. 1, makes an almost linearcorrespondence relation with the ferrite decarburization depth.

On the other hand, separately from the foregoing, each steel species wasmelt-formed, and subjected to slabbing and further to wire-rod rolling(13.5 mmφ) using a real machine at a rolling temperature of 900° C. Thecooling rate in this case was set to 0.5° C./sec. And an assessment ofan actual result of ferrite decarburization in each wire-rod rolledmaterial, namely a decision as to whether ferrite decarburizationoccurred (ferrite decarburization was present) or not (ferritedecarburization was absent), was made. The assessment results are shownin FIG. 1 in the form of coordinate data on every steel species withabsence of ferrite decarburization as a white circle and presence offerrite decarburization as a black circle. Additionally, in Table 1,absence of ferrite decarburization is described as “absent”, whilepresence of ferrite decarburization is described as “present”.

As can be seen from FIG. 1, it is appropriate to organize the ferritedecarburization depths into the X1 in Expression (1). And consideringthe actual results of ferrite decarburization under the practicalwire-rod rolling, it was determined that the threshold value of X1 fordecision as to whether or not ferrite decarburization occurred is 0.2.In other words, by adjusting X1 to 0.2 or less, it becomes possible toobtain structure free of ferrite decarburization.

-   (15) Satisfying the following Expression (2)

X2=(α−500)/β3 3.0   Expression (2)

α=912−231×[C]+32×[Si]−20×[Mn]−40×[Cu]−18×[Ni]−15×[Cr]

β=10̂(0.322−0.538×[C]+0.018×[Si]+1.294×[Mn]+0.693×[Cu]+0.609×[Ni]+0.847×[Cr])

In order to examine the adequacy of Expression (2), every steel specieswas, similarly to the above, subjected to slabbing and further towire-rod rolling (13.5 mmφ) using a real machine at a rollingtemperature of 900° C. In this case, cooling was carried out at twodifferent rates of 1.5° C./sec and 0.5° C./sec. And an assessment of anactual result of bainite formation in each of the wire-rod rolledmaterials, namely a decision as to whether bainite was formed (presenceof bainite formation) or not (absence of bainite formation), was made.Additionally, in Table 1 and FIG. 2, the unit of a cooling rate isexpressed in ° C./s.

The results obtained are shown in Table 1 and FIG. 2. FIG. 2 is a graphmade by plotting the coordinate data from every steel species withcooling rate as vertical axis and X2 in Expression (2) as horizontalaxis. Although X2 includes α and β as variables, the concept of theequality itself is known (for example, see Materia, vol. 36, No. 6,1997, pp. 603-608). α includes a polynomial formed by performingaddition or subtraction of component terms each of which is obtained bymultiplying each of the contents of the specified chemical components(C, Si, Mn, Cu, Ni and Cr) by the individually specified coefficient,and β is 10 to the power of such a polynomial. As shown in FIG. 2,considering the actual results of bainite formation under the practicalwire-rod rolling, it was determined that the threshold value of X2 fordecision as to whether or not bainite formation occurred is 3.0. Inother words, by adjusting X2 to 3.0 or more, it becomes possible toobtain bainite formation-free structures so long as the cooling iscarried out at usual execution rates.

-   (16) Satisfying the following Expression (3)

X3=31×[C]+2.3×[Si]+2.3×[Mn]+1.25×[Cu]+2.68×[Ni]+3.57×[Cr]−6×[Ti]≧24.0  Expression (3)

In order to examine the adequacy of Expression (3), steel samplesprepared by melt-forming individual steel species, hot-forging them intotheir respective bars having 22 mmφ and then machining them into theirrespective bars having dimensions of 20 mmφ×10 mm were kept at 950° C.for 60 minutes, subjected to oil quenching, then kept at 400° C. for 30minutes, and further tempered under air cooling. Hardness (HRC)measurements were conducted on the thus-treated steel samples.

The measurement results obtained are shown in Table 1 and FIG. 3. FIG. 3is a graph made by plotting the coordinate data from every steel specieswith hardness as vertical axis and X3 in Expression (3) as horizontalaxis. X3 includes a polynomial formed by performing addition orsubtraction of component terms each of which is obtained by multiplyingeach of the contents of the specified chemical components (C, Si, Mn,Cu, Ni, Cr and Ti) by the individually specified coefficient.

As can be seen from FIG. 3, it is appropriate to organize the hardnessinto the X3 in Expression (3). And in order for high-strength springsteel according to the invention to secure hardness (HRC) of at least53.0 in the case of setting the tempering temperature at 400° C., it wasdetermined that the threshold value of X3 is 24.0. In other words, byadjusting X3 to 24.0 or more, it becomes possible to obtain ahigh-strength structure with hardness (HRC) of 53.0 or more in the caseof setting the tempering temperature to 400° C.

-   (17) B: 0.0005% to 0.0050%

B is an element effective in improving a tenacity of spring steel bypreventing P and S from segregating to crystal grain boundaries.Therefore, the B content is preferably 0.0005% or more. On the otherhand, excessive addition of B causes formation of nitride of B, therebyresulting in degradation of the tenacity. Therefore, the B content ispreferably 0.0050% or less.

-   (Others) Reasons for setting the lower limit of Ti content to 0.060%

On the samples having undergone hot forging, subsequent quenching at950° C. and further tempering at 400° C., crystal grain size (austeniticcrystal grain size) measurements were made in accordance with theaustenitic crystal grain size testing method (JIS G 0551:2005). Themeasurement results (crystal grain size numbers) obtained are shown inTable 1 and FIG. 4. FIG. 4 is a graph made by plotting the coordinatedata from every steel species with crystal grain size number as verticalaxis and Ti content as horizontal axis.

The austenitic crystal grain size influences various characteristics (afatigue characteristic, a delayed fracture characteristic, a settlingproperty), and it is generally possible to improve these characteristicsthrough the fining of crystal grains. In the high-strength steel of thepresent invention, the lower limit of Ti content is set to 0.060 basedon FIG. 4 so that the crystal grain size after quenching-and-temperingbecomes No. 9 or more. In other words, by adjusting the Ti content to0.060% or more, it becomes possible to obtain fine structure which isNo. 9 or more in crystal grain size number.

Calculation results, measurement results and assessment results ofExpressions (1) to (3) corresponding to each steel species (in each ofExamples 1 to 12 and Comparative Examples 1 to 17) are shown in Table 1.As shown in Examples 1 to 12, high-strength spring steels havingexcellent wire-rod rolling properties, and more specifically, steelscausing neither ferrite decarburization nor bainite formation during thewire-rod rolling and having 400° C.-temper hardness of 53.0 or more anda crystal grain size number of 9 or more, can be obtained by adjustingeach of chemical components to fall within the individually specifiedcontent range and satisfying Expressions (1) to (3).

On the other hand, in each of Comparative Examples 1, 6, 10, 11, 14, 15and 17, Expression (3) was not satisfied; as a result, the 400°C.-temper hardness was below 53.0 HRC. Additionally, in each ofComparative Examples 4 to 11, Expression (1) was not satisfied; as aresult, ferrite decarburization occurred during the rod-wire rolling.

Further, in each of Comparative Examples 2, 3 and 15 to 17, the Ticontent was below 0.060 mass %; as a result, the crystal grain sizenumber thereof became below No. 9. Furthermore, in each of ComparativeExamples 10 and 12 to 14, Expression (2) was not satisfied; as a result,bainite formation occurred during the wire-rod rolling.

As can be clearly seen from the above descriptions, according to thepresent invention, it is possible to obtain a high-strength spring steelhaving excellent wire-rod rolling properties. Incidentally, the presentinvention should not be construed as being limited to the foregoingExamples, but can be carried out in modes undergone various changes andmodification so long as they do not depart from the gist of theinvention.

The present application is based on Japanese Patent Application No.2014-206311 filed on Oct. 7, 2014, and the contents are incorporatedherein by reference.

What is claimed is:
 1. A high-strength spring steel having excellentwire-rod rolling properties, consisting essentially of, in terms of mass%: C: 0.40% to 0.65%; Si: 1.20% to 2.80%; Mn: 0.30% to 1.20%; P: 0.020%or less; S: 0.020% or less; Cu: 0.40% or less; Ni: 0.80% or less; Cr:0.70% or less; Ti: 0.060% to 0.140%; Al: 0.10% or less; N: 0.010% orless; and O: 0.0015% or less, and optionally: B: 0.0005% to 0.0050%,with the remainder being Fe and inevitable impurities, wherein thecontents in terms of mass % of the specified chemical components satisfythe following Expressions (1) to (3):X1=0.14×[ Si]−0.11×[Mn]−0.05×[Cu]−0.11×[Ni]−0.03×[Cr]+0.02≦0.2  Expression (1)X2=(α−500)/β3.0   Expression (2)α=912−231×[C]+32×[Si]−20×[Mn]−40×[Cu]−18×[Ni]−15×[Cr]β=10̂(0.322−0.538×[C]+0.018×[Si]+1.294×[Mn]+0.693×[Cu]+0.609×[Ni]+0.847×[Cr])X3=31×[C]+2.3×[Si]+2.3×[Mn]+1.25×[Cu]+2.68×[Ni]+3.57×[Cr]−6×[Ti]≧24.0  Expression (3).
 2. The high-strength spring steel having excellentwire-rod rolling properties according to claim 1, having a 400°C.-temper hardness of 53.0 HRC or more.
 3. The high-strength springsteel having excellent wire-rod rolling properties according to claim 1,having a grain size number of 9 or more.
 4. The high-strength springsteel having excellent wire-rod rolling properties according to claim 2,having a grain size number of 9 or more.